Studies suggest that Mexican migrants and immigrants (MMIs) in the US are at increased risk for HIV infection and face important barriers to access health care services. Studies have estimated that HIV infection is over 10 times more prevalent among MMIs compared to the overall Mexican and US population and suggested that MMIs serve as transmission vectors for increasing rates of HIV/AIDS in rural Mexico. Most research on this topic has been limited by methodological difficulties to reach this mobile, geographically widespread, and often undocumented population. Few conclusive data exist regarding the true prevalence of HIV infection, related risk practices, and determinants of HIV risk among MMIs, including the role of exposure to US society and limited health care access. This binational study will investigate the prevalence, patterning, and determinants of HIV infection and related risk practices among a representative sample of the MMI population who travels through the San Diego - Tijuana border region. This region concentrates about 37% of the migrant flow traveling through the US-Mexico border and represents the natural crossing port between Mexico and California. Survey methods that have been successfully applied to characterize the socio-demographic, labor, and geographic patterns of the MMI population in the North border of Mexico for over 13 years by Mexican investigators at El Colegio de la Frontera Norte (COLEF) will be used. A cross-sectional, multi-stage probability survey on HIV and migration will be conducted at key migrant crossing sites in the border city of Tijuana (Baja California, Mexico). MMIs (N=3,800) representing four distinct subpopulations at different stages of the migration experience will be tested for HIV infection and surveyed on HIV-related practices and theoretical determinants. The sample will include MMIs (a) returning voluntarily from the US to Mexico; (b) returned from the US to Mexico by the US Border Patrol; (c) arriving at Tijuana from other Mexican border regions; and (d) traveling North from other Mexican regions. Following the Behavioral Ecological Model, the role of demographic, geographic, economic, social, and psychosocial factors on HIV risk practices in these four subpopulations and the changes in these factors associated with different contexts and stages of the migration process will be examined. In addition, a pilot survey (N=300) on health care access and utilization will be conducted to test the feasibility of applying the proposed survey methods to the study of this and other migrant health areas. Results from this study will advance behavioral epidemiology and may lead to actions in Mexico and the US to reduce HIV infection and improve health care access among MMIs. This study may also set the basis for binational collaboration in the monitoring of migrants' health and provide baseline data to evaluate the effectiveness of future policies aimed at reducing HIV risk and health disparities in both countries.